The present invention relates to polyolefin films and biaxially oriented films with improved properties useable as shrink films. In particular, the present invention relates to shrink film for use in packaging lines, although the invention may also be used in other shrink film applications.
Polyolefins, polyvinyl chlorides, ionomers, polyesters, polystyrenes, and polyvinylidene chlorides have all been used in the production of shrink film. The shrinkable polyolefins currently on the market include both cross-linked and non cross-linked oriented polyethylene, oriented polypropylene, and oriented ethylene-propylene copolymers.
A shrink film's distinguishing characteristic is its ability upon exposure to heat to either shrink or, if restrained, to create shrink tension within the film. This ability is activated by the packager when the wrapped product is passed through a hot air or hot water shrink tunnel. This process causes the film to shrink around the product producing a tight, transparent wrapping that conforms to the contour of the product and which is aesthetically pleasing while providing the useful functions required of packaging materials such as protection of the product from loss of components, pilferage, damage due to handling and shipment, dirt and contamination.
Typical items wrapped in polyolefin shrink films are toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
The manufacture of shrink films requires sophisticated equipment including extrusion lines with “biaxial orientation” capability. Biaxial orientation can be accomplished with “tenter frame” stretching or “double bubble” blown film extrusion processes. The biaxial orientation causes the material to be stretched in the cross or transverse direction and in the longitudinal or machine direction. The films are usually heated to their orientation temperature range that varies with the different polymers but is usually above room temperature and below the polymer's melting temperature. After being stretched, the film is rapidly cooled to quench it thus freezing the molecules of the film in their oriented state. Upon heating, the orientation stresses are released and the film will begin to shrink back to its original unoriented dimension.
The polyolefin family of shrink films provide a wide range of physical and performance characteristics such as shrink force (the amount of force that a film exerts per unit area of its cross-section during shrinkage), the degree of free shrink (the reduction in surface area a material undergoes when unrestrained), tensile strength (the highest force that can be applied to a unit area of film before it begins to break), sealability, shrink temperature curve (the relationship of shrink to temperature), tear initiation and resistance (the force at which a film will begin to tear and continue to tear), optics (gloss, haze and transparency of material), and dimensional stability (the ability of the film to retain its original dimensions under all types of storage conditions).
In addition to the above, high speed automatic wrapping lines require that the shrink film have high film-film slip (low coefficient of friction), consistently strong static seals, higher stiffness compared to other shrink films and preferably good low temperature shrink initiation (water bath shrinkage is used to simulate low temperature shrink initiation). The strong static seals allow for fast sealing of the packages. Higher stiffness supports high film and packaging line speeds. Stiffness is measured as Young's modulus or “modulus” per ASTM test D 882. Good low temperature shrink initiation is advantageous in permitting lower shrink tunnel temperatures and having reduced shrinkage times (thereby permitting shorter shrink tunnel times).
High slip allows packages to freely pass one another on conveyors without sticking or clinging. Slip may be determined at room temperature or at elevated temperatures. Slip at room temperature is representative of conditions packaged goods encounter during transportation between facilities and during loading and off-loading of packages from transport vehicles.
Packages having high slip at room temperature may still have poor hot slip characteristics. Hot slip is important for operating packaging lines at high speeds. One determinant in line speed is the amount of time required to cool the packages downstream of the shrink oven or chamber. Packages must be cooled to a point where they no longer cling to other packages. For this reason, packages with superior hot slip require less cooling time and therefore allow faster packaging line speeds.
One approach to shrink films for high speed wrapping lines is a multilayer film comprising polyethylenic and polystyrenic layers. The differences between polystyrene and polyethylene layers require intervening compatabilizing layers for successful manufacture. U.S. Pat. No. 6,376,095 to Cheung et al. discloses random interpolymer useful for a compatabilizing layer. Such random interpolymers include the ethylene/styrene interpolymer resins prepared using a constrained-geometry catalyst. Unfortunately, Cheung compositions are presently expensive due to the cost of the interpolymer and may be subject to phase-out by the manufacturer. Films of this genus will hereinafter be referred to as “polystyrene layered.”
A similar approach to shrink films using polystyrenic layers are the films disclosed in U.S. Pat. No. 6,479,138 to Childress. The Childress films have a modulus in excess of 40,000 psi and a haze value less than 6. However, the Childress films have the similar problem as the Cheung films in relying upon relatively expensive components. Both the Cheung and Childress films have established a commercial expectation to shrink films for high speed packaging lines having a modulus of 40,000 psi or higher and a haze value less than 5.0.
Another approach to films is use of cyclic-olefin copolymers (COCs). COCs are available commercially from Ticona of Summit, N.J. For example, U.S. Pat. No. 6,068,936 to Peiffer discloses films comprising polyolefins and cyclic-olefin copolymers. However, the Peiffer patent discloses cast films, normally used as stretch films or pouch films, and limits all examples to polypropylene based polyolefins.
Accordingly, it is a general object of the present invention to provide a shrinkable non-polypropylene film having improved properties for high speed packaging lines comparable to those of the polystyrene layered shrink films but without the attendant costs of compositions for compatabilizing layers of such films and with superior hot slip characteristics.